Process for making silicon metal

ABSTRACT

THERE IS DESIRED AN IMPROVEMENT IN MAKING POLYCRYSTALLINE SILICON METAL, AS USED FOR THE MAKING OF SEMICONDUCTORS, FROM TRICHLOROSILANE WHICH INVOLVES PROVIDING GREATER AMOUNTS OF TRICHLOROSILANE IN A METAL FORMING REACTOR THEN HERETOFORE HAS BEEN EMPLOYED. THIS PROVIDES GREATER SI METAL PRODUCTIVITY THAN WAS HERETOFORE BELIEVED POSSIBLE.

United States Patent 3,809,571 PROCESS FOR MAKING SILICON METAL Norman R. Berlat, Vienna, W. Va., assignor to Union Carbide Corporation, New York, NY. No Drawing. Filed Apr. 4, 1972, Ser. No. 241,085 Int. Cl. C23c 11/06 US. Cl. 117-106 A Claims ABSTRACT OF THE DISCLOSURE There is described an improvement in making polycrystalline silicon metal, as used for the making of semiconductors, from trichlorosilane which involves providing greater amounts of trichlorosilane in a metal forming reactor then heretofore has been employed. This provides greater Si metal productivity than was heretofore believed possible.

The invention relates to the manufacture of polycrys- 'alline silicon metal by the thermal reduction of trichlorosilane, and, more specifically, this invention involves an improvement in silicon metal production from the ther- 'mal reduction of trichlorosilane to achieve higher silicon metal productivity than heretofore thought possible.

Polycrystalline silicon metal is commercially produced by the thermal reduction of either trichlorosilane, silicon tetrachloride or silane. Practically all silicon metal commercially made is obtained by the thermal reduction of trichlorosilane (HSiCl in a closed reactor in which there is suspended, out of contact with the reactor walls, a deposition rod (the term rod as used herein and in the claims includes and encompasses rods, wires, filaments, etc.) which is electrically heated, usually by direct current. The most common reactor has an ordinary bell jar design, comprising a metal base plate from which the rod is extended upwardly, inletsand outlets therein for gases, and a quartz bell. jar cover over the plate. Electrical connections are provided in the reactor for heating the rods.

The rod is brought to reaction temperature, usually from about 1000 C. to about 1200 C., and then HSiC1 mixed with hydrogen gas is fed to the reactor. Most of the heat is about the rod surface and therefore, silicon metal production occurs on the rod surface. It is important, as in most chemical reactions, to avoid reactions on the reactor wall(s) and, hence, the wall(s) is kept at a temperature sufliciently low to preclude such reactions thereat, particularly of the kind which produces a silicon metal coating on the wall(s) The trichlorosilane fed to the reactor with H gas is mixed with the H in the usual case, by bubbling H through or passing H over a liquid or vaporous body of HSiC1 The temperature of the 'HSiCl and the temperature of the gas mixture, determines the partial pressure of HSiCl and, therefore, the mole percent of HSiCl in the mixture. In the usual case, not much more than 5 mole percent HSiCl is provided in the mixture, the remainder being hydrogen.

The flow rates, mole ratios and reaction conditions are established at the beginning of the reaction, and the production run is typically carried out without significant variation.

The nature of the reduction of HSiCl has been chemically examined, and certain factors from such examination have led to the discovery that silicon metal production can be, from a productivity standpoint, greatly enhanced, indeed, can be at least doubled, and even increased to a rate at least five times greater. The following are now appreciated to be determinants to Si metal production. (A) The deposition of silicon metal from trichlorosilane has been determined to occur within 0.3 second at 1000 C. probably by 0.1 second. This means 3,809,571 Patented May 7, 1974 that the decomposition of HSiCl occurs faster. (B) It is now established that HSiCl disproportionates in space at such deposition temperatures with HCl and SiCl to form a silicon compound mixture containing less hydrogen per Si than HSiCl and the nature of the disproportionated products are dependent upon the proportion of HCl to SiCl to HSiCl (C) It is known that reduced silanes convert to Si metal faster than fully oxidized silanes such as SiCl viz, compare H Si, H SiCl, H SiCl HSCl and SiCl (D) It is logical to assume that in a silicon metal reactor, the concentration of SiCL; is greater at about the rod surface where it is formed and, therefore, the H to Si ratio at about the rod surface is lower than at any other portion of the reactor. (E) Also it has been determined that the temperature gradient from the hot rod to the much cooler reactor wall drops significantly at about a few centimeters from the rod to a temperature below the effective decomposition temperature of HSiCl It therefore follows:

(1) Most, if not essentially all of the reactions, spacial and/or surface, occur about the rod surface;

(2) The concentration of HSiCl at about the rod surface to provide a high H/Si ratio is most important to the effectiveness of the process.

It is now known that the rate of silicon metal growth in the known HSiCl silicon metal processes can be at least doubled, and in some cases can be increased by 500 percent or more, by the simple expedient of insuring that enough HSiCl is present in the reactor to achieve and maintain that growth rate. The use of such large amounts of trichlorosilane in the reactor coupled to the consequent rate of the increase in rod surface area is novel. It is believed that the increased amount of HSiCl in the reactor achieves a H to Si ratio at about the rod surface(s) more favorable to silicon metal production.

There is described herein an improved process by which it can now be shown that the prior art and prior commercial practices relating to the manufacture of Si metal from HSiCl describe or operate starved reactions, i.e., employ less HSiCl than is described herein in the same apparatus and with the same process steps.

The process of this invention involves using the prior art techniques of making Si metal from HSiCl -H gas mixtures characterized by the improvement which is to maintain in the reactor greater amounts of trichlorosilane wherein A is defined as grams per hour of HSiCl maintained in the reactor (gm./hr.); G is the growth of the rod during the process in grams per hour per square centimeter (g./hr./cm. and desirably exceeds about 0.125 g./hr./cm. and usually does not exceed about 0.5 g./hr./cm. preferably at least about 0.2 g./hr./cm. to about 0.4 g./hr./cm. R is the surface area of each rod in the reactor and is the sum of the surface areas of each rod in the reactor measured in square centimeters (cmfi); and C is the fraction conversion of HSiCl to silicon metal. The constant 4.8237 is the molecular weight of HSiCl (135.5) divided by the atomic weight of silicon (28.09).

The amount of HSiCl desirable in the reactor pursuant to the above equation is not dependent upon a concentration factor. However, concentration of HSiCl in the feed to the reactor, i.e., the HSiCl -H gas mixture, can be employed as a means to provide the desired amount of HSiCl in the reactor. The second method for controlling the HSiCl content in the reactor is by increasing or decreasing the feed rate of HSiCl -H mixture to the reactor. A combination of concentration and feed rate represents the third way of controlling or providing the proper amount of HSiCl in the reactor.

4 thereof. The bell jar cover contains a rectangular glass port having the dimensions of 2 inches by 12 inches located near the open end thereof for the purpose of observing the reaction. The bell jar is placed upon a base plate constructed of carbon steel with a nickel and iridium Because the amount of HSiCl present in the reactor, coating. The base plate 15 water cooled in the convenaccording to this invention, is greater than heretofore tional manner to prevent warpage during the reaction. thought feasible, limiting the concentration of HSiCl in The base plate also contains two water cooled silver electhe gas feed to the accepted maximum of about 5- mole trodes connected to a power supply which provides the percent is no longer valid. Hence, the skilled worker can high voltage-low amperage starting current for silicon now employ, according to this invention, much greater rods suspended on the base plate and within the bell jar concentrations of HSiCl as much as 25 mole percent 'of cover. The base plate contains an inlet nozzle at its centhe gas mixture ifeed. But since high concentration is not ter having a /2 inch outside diameter and a inch a critical factor to achieving the requisite amount of inside diameter which protrudes 3 inches above the base HSiCl in the reactor, concentrations of about 2 mole per- P into the interior area of the bell j ctWeY- At one cent, or lower, of HSiCl in the feed may be employed. eide of the base plate is an outlet nozzle having a 2-inch In the most desirable embodiment of this invention, the nside diameter. Each silicon rod suspended in the reactor amount of risici provided in the reactor exceeds 4 mole held in spectroscopie grade carbon sockets which are percent f the gases in the reactor, and f rabl exceeds directly connected to the silver electrodes. These sockets 5 mole percent of the gases in the reactor. Since the invenare Called A Spectroscopic grade Carbon rod is tion includes introducing trichlorosilane from recycle fitted across the ends of every tWo rods to thereby inter gases, small amounts of by-products produced in the re Connect theIh t0 comPlete Passage 0f the ellffent through actor and not thereafter removed in a normal processing the upstahdlhg rods. T bell 'i cover slts p a seal and separation, such as dichlorosilane, monochlorosilane, located about the P P e Y 0f the belse Plate to Prevent and Shane may be present leakage of gas from the interior of the reactor. The out- In a desirable embodiment, trichlorosilane is positively slde 0f the reactor contams metal Shield as a PI'OteetiVe fed into admixture with hydrogen so as to insure direct Shroudcontrol over the amount of trichlorosilane which is pres- All hydrogen which 18 i to the reactor 13 Controlled ent in the feed gas mixture. The temperature of the by rotometers. Trichlorosilane used in the experiments action is essentially that which is defined above and which 1S VaPOTIZed from a laeketed Yessel Whleh 1S heated y is practiced by the are hot water passed through the acket and the amount of The rate of growth of the rods during the reaction can trichlorosilane vapor fed is controlled by a rotometer. be easily determined by noting the increase in the heme The trichlorosilane and hydrogen gases are mixed prior ter of the rod during the decomposition reaction. Since to entenhg the reactor through h Inlet nozzle- The the length of rod is fixed, the diameter increase will Pqnems Introduced through the mlet 110F218 and removed directly yield information as to the growth rate on a' gram from the outlet nozzle F analyzed m a vapor Phas? per square centimeter basis. They will provide instant chromafograph; Before mtroductlon hydrogen? information [for calculating the area growth rate as specichloroshane mlxturet, to the reactor 1t Sparged Wlth fled by the equation ahove nitrogen gas for 15 minutes to remove any oxygen present In practicing the process of this invention one must give 40 and thereafter h h Wlth hydrogen gas for concern with growth of reduced trichlorosilane upon the 15 mmhtes than the HSCL? 15 added to hydfogeh interior of the reactor wall, particularly silicon metal degas and both are fed to h reactor as speclfihd m the composition on the reactor wall. This is a problem partables below The power 15 to the rods and once ticularly when the size of, the rod increases so much as to t are brought to the deshed temperature the increase the temperature at the wall of the reactor by Input 15 cohtrohed Ihahhahy The temperature. 18 virtue of the close proximity of the rod surface to the measured Wlth an ophchhpyromethr dunhg the dhrahoh reactor wall. The wall reactor temperature can be reduced h qhahhty mchloroshahe pmvlded by increasing the velocity of the gas introduced to the W1 {hm the rehctor 1S hshahy increased ashle Surface area interior of the reactor. If this technique is employed to g rods Increases Thls lhsures maxmhhh growth of reduce the wall temperature, care should be taken that ives the owth t and t 1d t236 the amount of trichlorosilane introduced into the reactor g Ia es Percen yle s a 472,708 and 944 hters per minute. As expected, the

at that point and time does not end up wasting trichlorogrowth rates increased with higher mole ratios and the silane by reducing the conversion rate (C) to an unacce tabl lev 1 In h th t f percent yield decreased with increased mole ratio.

hTl e sue cases, 6 collecen m O The surface quality dramatically improved with mole c dorosglane 1n the y g teed mlxtule Should be ratio. At two mole percent, the surface of the silicon was Te rough and cobby throughout. As the mole ratio was in- III the tables Whleh follow, the feactmhs ep therecreased, the crystalline structure became more uniform in were carried out in a conventional bell ar-type polyand smooth. crystalline SlllCOH reactor of the following descriptioii: In each experiment indicated in Table I, the total length An opaque quartz bell ar cover having an inside diameof the run was six hours. The reactor in each run conter of 17 inches, a wall thickness of A of an inch, and a tained two upstanding rods and the rod temperature was height of 40 inches to the top of the hemisphere dome 1150 C.

TABLE I Effect of mole ratio trichlorosilane upon silicon growth rate Average growth rate Experiment H flow, Hsioi HS'Cl M 1 sin d number 1 ./min. kgJhi. LIiniii. ra ti gr r i s tei gi l d Gms.lhr. iiii e iih 236 4.09 10.9 4.6 2 236 5. 685 15. i 6. 0 242 3i? 3. i is: 2 ii 236 9. 0s 25. 5 9. 7 792 1. 499 7. 0 132 190 0' 290 472 3. 6 10. 4 2. 1 554 1. 270 11. s 92 14' s 0' 230 472 9. 0s 25. 7 5. 2 842 1. 588 7. 3 140 19' 2 0' 291 472 12. 7 35. 2 6. 9 1, 000 1. 676 6. 9 166 20' 5 0' 314 472 18. i6 50. 3 9. 6 1, 020 1. 689 5. 2 169 20' 6 0' 320 s 5. 45 15. 1 2. 0 574 1.422 s. 25 96 16' 9 0 209 70s 11.35 30. 2 4. 0 970 i. 681 7. 35 162 20' 5 0' 300 70s 16. 34 45.3 6. 0 1, 1. 821 6. 02 22' 5 0' 341 944 is. 16 50. 5 5. 0 1, 36s 1. 976 6. 09 237 25' 6 0' 381 944 45. 4 106. 7 11. 7 1, 366 1. 94s 2. 9 273 29: 5 0: 440

perature and mole ratio. This illustrates that the amount of trichlorosilane in the reactor vapor space -1s very I The rod diameter increased mm 1.427 cm. to 3.411 cm'.-and"the absolute growth rate increased from 168 grams/hour to 309 grams/hour. By taking the incremental weights for the incremental time, the incremental 'growth'rates were obtained. The absolute growth rate was 472 grams/hour at the end while the unit growth rate leveled off at 0.23-0.25 grams/hour/cmfi. The rods were leaning: very badly and in several runs actually touched the reactor wall. However, when the silicon rod continues to grow at microns/minute it is possible to grow two four-inch rods in 48 hours-heretofore, an

important. ofproduction rate.

1 ;g .TABLEn I Efiect 01H: gas flow upon silicon growth rate A e m t n a i P t t ri iii Rod di v rage grow m g '1 ow ow ercen per re s con am- Percent Micron Gms. r. Experiment number l./min: LIminZ mole C: gms: eter, cm yi GmsJhr. min! 236 11.8 4.9 1,150- 526- 1.245 10.6 .88 14.5 0.224 472 25.5 5.2 1,150 842 1.588 7.3 140 19.2 0.281 '708 45.3 6.0 1,150 1,120 1.821 6.0 195 22.5 0.341 944 50.0 5.0 1,150 1,368. 1. 976{ 6.0 238 25.6 0.381 472 10.4 2.0 1,150 554 1.270 v 11.8 92 14.8 0.230 708 15.1 2.0 1,150 574 1.422 8.2 96 16.9 0.209

One would expect that the higher gas flows would sufficiently lower the residence time to reduce, the reaction. Although the yield decreases by two,--the absolute quantity of silicon fed to the reactor increases by four; ,afnet increase in growth rate. The high gas velocities increased the turbulence and the heattransfer characteristics of the system. The efiluent gas temperatures at the end of-the run were 245 C. and 350 C. for-the 236 liters/minute and 944 liters/minute, respectively. E

In Table III, four rods were grown to compare to 'two rods. As expected, the doubling of the siliconsurface area had an appreciable eifect.

In order to maintain straight silicon rods the initial Qrod temperature was held at 1100 C. and the hydrogen .flow at .472 liters/minute until the rod diameter was large enoughto prevent vibration. Then the rod temperature was increased to 1150 C. The run was terminated after 36 hours due. to electrical problems. The rod diameter was 6.985 cm. and the absolute growth rate was 472 grams/hour.

The operating conditions were varied throughout the TABLE III HSibh Rodtem- Weight .i- I Rod Experiment Hzflow flow, j Percent peratu're, silicon, 1 Silicon .-.diameter, Silicon, number Llmin. l./mi n. mole C. gms. wi l Gnigms-lhr. 13 201 14.2 I 5.1 1 100 693 5 10.9' 1.054 nus 14 23s 1 .21 5.7 1I1oo 360 j 5.6-

. 4rods. 2rods. r

The quantity of silicon obtained was; practically doubled. I

while the rod sizes were identical. The silicon yield also, doubled. The silicon surface on the four-{rods showed some initial cobbiness at the base but the femainderwas smooth. The surfaceof the two'rod setupwas'smooth throughout.

A twelve'hom' pfl lt Wasrun under the following conditions:

H flow: 708 liters/minute Rod temperature: 1150 C. HSiCl 18.16-22 kg./hour Table IV shows five experiments attempting todeposit silicon for twelve hours. At the high gas flows 'and'rod temperature the silicon rods vibrated. Thrice vruns were terminated prematurely, before the end of twelve hours, when the rods broke. 5

" when fog (either. silicon metal or polysilanes) appeared on the reactor walls, the flow rate of HSiCl was reducedgWithin' 30-60 minutes, the reactor wall haze disappeared. If the HSiCl how was increased to the original va1u e,f fog "wouldfreappear. Therefore, when the HSiCl; flow was increased the rod temperature was lowered in 25 C. increments. If fog developed, the hydrogen flow was increased in 250 .feet /honr increments. -At the time-of shutdownthe rod temperature was 1 100? .C. and thehydrogen-fiow was 708 liters/min.

Table :V shows-:thetrichlorosilane requirements to grow two four-inch rods within 48 .hours. As the surface area 'to prevent starvation.

increases-more trichlorosilane must be fed to the reactor TABLE IV Extended runs with trichlorosilane' Ru Aveisg'e ibwm me n Experiment time, Silicon, Rod dia- Percent Silicon, Micron! Gms.lhr./ Weight, Time, Gms./hr./ number hours wt.-gms. meter, cm. yiel gmsJhr. min. cm. gms. hrs Gms./hr cm.

Conditions: 2 rods; H2 flow, 708 liters/minute; Rod temperature, 1,150 0.; HSiClz, 18.1-22.7 kg./hr., 50-626 l./min.; Mole ratio, 6.6-8.1.

.7 8 would plateau at 800 grams/hour after. the rods reached .tained in the reactor (gm./hr.); G is the growth of the two inches in diameter. The rods would then grow starved. rod during the process in grams per hour per square cen- TABLE v y timeter and from about 0.125 g./hr./cm. to about 0.5 g./

hr./cm. R is the sum of the surface areas of each HSiCli requirement versus rod diameter (2 rods) 5 rod in the reactor measured in square centimeters (cm.

Assume Assume growth mmmum Tncmow and C is the fraction conversion of HSiCl to silicon Surface rate, 0.25 conversion siiane t 7 area, gms./ HSiCls '00 required, Diameter, mm mm swoon- 25% kgJl 2. The improved process of claim 1 wherein G is at 1,600 400 1,600 a0 least about .2 g./hr./cm. and not greater than about 3,200 600 3,200 16.0 .4 g./hr./cm. 4, 600 1, 200 4, 600 24. 0 6,400 1,690 6,400 3m 3. The process of claim 1 wherein there are two rods in 4 a the reactor. The following table is a compilation of experimental 4. The process of claim 1 wherein there are four rods runs showing a variety of experimental conditions and in the reactor. results thereofsome i 9. x erimen w re 5. The process of claim 1 wherein gases from the reac previously cited. tion are recycled to the reactor.

TABLE VI- Rod Average growth rate Num- Length temper- H, 118101 HSiCI; Weight Percent Bod di- Experiment ber of of run, ature, flow flow, flow, Mole silicon, converameter Micron] Gms./hr/. Number rods hrs. C. llmin. llmin. kgJhr. ratio gins. 61011 cm. Gms./hr. min. 0111.

2 6 1,100 352- 123 -4.4 3.4 380 6.9 1.060 63 12.2 0.175 2 3% 1, 100 405 15. 1 5. 4 3. 6 411 9. 7 1.161 66 34. 7 0. 277 2 6 1,100 200, 15.6 5.7 7.4 420 6.0 1.060 70 12 0.136 2 6 1, 100 70' 17. 9 6. 4 20. 6 435 5. 4 1. 105 72 12. 3 0. 196 4 6 ,100 207 14.2 5.0 6.4 693 10.9 1.054 115 11.6 0.172 4 6 1, 100 202 13.2 4. 5 6.1 556 10. 4 0. 914 9. 9 0. 153 2 6 1,100 250 6.5 5 2.6 2.6 312 10.7 0.965 10.6 0.134 2 6 1,100 236 14. 2 5. 0 5. 7 360 5. 6 1. 041 11.6 0.172 2 6 1,150 236 15.1 5.7 6.0 64 9.1 1.397 16.6 0.242 2 4. 5 1, 200 236 17. 0 5. 9 6. 7 542 9. 6 1. 257 19. 5 0. 263 2 6 1, 200 65 23. 4 13.1 21. 7 622 7. 6 1. 613 19.5 0. 270 2 4. 5 1, 200 221 21. 7 7. 7 9. 0 560 7. 7 1. 295 30. 2 0. 304 2 6 1, 150 236 25. 6 9. 1 9. 7 792 7. 0 1. 499 16 0. 260 2 6 1, 150 236 10.9 4. 1 4. 6 526 10. 6 1. 245 14.5 0. 224 2 6 1,100 472 21.9 7. 9 4. 4 602 6. 3 1. 306 15. 4 0. 243 2 6 1, 150 472 25. 7 9. 4 5. 0 642 7. 3 1. 566 19. 2 0. 261 2 6 1, 150 472 35. 2 12.7 6. 9 1, 000 6. 9 1. 676 20. 5 0. 314 2 6 1, 150 472 50.5 16. 1 9. 6 1, 020 5. 2 1. 669 20. 6 0. 320 2 6 1, 150 472 10. 3 3. 6 2. 1 554- 11. 6 1. 270 14. 6 0. 230 2 2 1,150 706 17.5 6.4 2.4 156 6.0 0.701 26.4 0.319 2 Aborted Aborted 2 6 1, 150 706 15. 1 5. 4 2. 0 574 6. 1. 422 96 16. 9 0. 209 2 6 1, 150 706 30. 2 11.4 4. 0 970 7. 1. 661- 162 20. 5 0. 300 2 6 1,150 708 45.3 16.3 6.0 1,120 6.02 1.621 195 22.5 0.341 2 6 1,200 472 22.6 6.2 4.6 940 6.90 1.694 p 157 20.7 0.295 2 6 1, 050 472 22. 6 6. 2 4. 6 300 2. 95 0. 940 10. 2 0. 169 2 6 1, 150 472 43. 9 15. 9 6. 6 1, 060 5. 37 1. 727 177 20.4 0. 325 2 5 1,130 125.6 45.4 11.7 1,366 2.9 1.946 273 0.446 2 5% 1, 150 50. 0 18.1 5. 0 1, 368 6. 0 1. 976 236 25. 6 0. 361 2 3% 1,175 706 50.5 16.1 5.0 I 516 5.6 1.209 159 25.6 0.417 2 6.5 1,150 706 64.1 22.7 6.3 1,300 4.6 1.621 193 20.7 0.313 2 8% 1,150 706' 65.3 22.7 6.4 2, 010. 4.6 2.515 229 22 0.322 2 4 1, 150 708 56. 6 20. 4 7. 4 674 4. 0 1. 426 166 25 0. 374 2 13 1,150 706 66.9 25.0 6.9 4,020 4.6 3.411 309 20 0.309 2 13 1,150 472 70.6 25.0 13.3 3,696 5.26 3.216 264 19.3 0.260 2 5 1,125 706 Varied 416 3.64 1.122 63 15.3 0.236 2 16 1, 150 706 Varied 4, 600 6. 57 3. 656 266 16 0. 250 2 36 Conditions varied throughout run 17, 025 9. 76 6. 934 472 15. 6 0. 216 2 16 Conditions varied 3, 005 7. 26 2. 946 166 14.2 0. 203 2 6 1,100 377 1,021 4.1 1.706 127 15.6 0.237 2 4 1, 075 472 23. 6 4. 8 262 3. 7 0. 669 66 14. 3 0. 234

What is claimed is: References Cited In the process of continuously making polycrystall ne UNITED STATES PATENTS silicon metal on a rod deposition surface in a reaction by the continuous thermal reduction at a temperature of 3,748,169 7/1973 117106 A about 1000 c. to about 1200 c. of HSiCl in admixture 3,523,816 8/197 Cave with hydrogen, wherein the improvement comprises main- 3679470 7/1972 et 117-406 taining an amount of trichlorosiiane in said reactor which 3635683 1/1972 Han-son et 117 106 satisfies the equation: Y

- ALFRED L. LEAYITI, Primary Examiner 3 1 I. MASSIE, Assistant Examiner C 4 US. Cl. X.R.

v 117-1351 wherein A is defined as grams per hour of HSiCI 'main-' Patent No. 3,809,571 Dated May 7, 1974 Inventor) Norman R. Berlat It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 14, "then" should read than Column 4, line 53, "ratios" should read percent Column 4, line 54, "ratio" should read percent Table I, line 65, "ratio" should read percent Table I, line 68, "ratio" should read percent Column 5,, line 9, "ratio" should read percent Table IV, last line, "ratio" should read percent Table VI, column 8, "ratiomghould read percent Table VI, column 1, after "Experiment Number" delete l Table VI, column 6, first line of numbers, "123" should read 12.3

Table VI, column 1, "'29" should read l9 "'11" should read 2l "23" should read l2 "13'' should read 23 "24" should read l4 "36" should read 26 1" should read 3 ",2" should read l "57" should read 27 6" should read 5 7" should read 6 4" should read 7 2" should read 4 "89" should read 29 9" should read 8 1" should read 9 "30" should read l0 "12" should read 32 "31" should read ll Pat 212 4. 7

, UNITED STATES PATENT OFFICE i I CERTIFICATE OF CORRECTION 1 Patent 3.809.571 Dated May 7, 1974 Page 12 Inv fl Norman R. Berlat It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Table VI, column l,' "14" should read "34-- "35" should read --l5-- "15" should read --35-- I "36" should read --l6-- v "18" should read --38-- 7" should read -'-l7--.

Column 7, claim 1, line 56,- "reaction" should read "reactor".

I I I Signed and sealed this 4th day of March 1975.

(SEAL) Attest:

a i C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks- 

